1. Field of the Invention
The present invention relates to a continuously variable transmission for bicycles, and more particularly to a continuously variable transmission including a plurality of link units for converting rotary motion of an input shaft into swinging motion, a one-way clutch for converting the swinging motion into rotary motion of an output shaft, and a transmission control mechanism for continuously varying the rotational speed of the output shaft.
2. Description of Background Art
There has heretofore been known a continuously variable transmission for bicycles of the type described above, as disclosed in Japanese patent laid-open No. Sho 54-120146. This continuously variable transmission, which is disposed near the axle of the rear wheel, has a crankshaft rotatable by a torque transmitted from a drive sprocket of a pedal through a chain to a driven sprocket, a crank rod pivotally supported on an eccentric pin of the crankshaft, a one-way clutch disposed on a countershaft, and a feed rod having an end pivotally mounted on a drive member of the one-way clutch and another end pivotally mounted on the crank rod. Also included are an output shaft to which rotation of the countershaft is transmitted, a coupling pin coupling the crank rod and the feed rod to each other, a turning member with which a pivot shaft is integral, and a link having an end pivotally mounted on the coupling pin and another end pivotally mounted on the pivot shaft. The turning member is swung to change the position of the pivot shaft for thereby continuously varying a speed change ratio.
According to the above reference, since the transmission which is a heavy object is positioned near the axle of the rear wheel which is spaced from the center of gravity of the bicycle body, the maneuverability of the bicycle is poor. Furthermore, since a portion of the transmission is positioned below the axle, it may possibly contact the ground when the bicycle travels on rough terrain. If the drive sprocket which also serves as a speed increasing mechanism for increasing the rotational speed of the crankshaft of the continuously variable transmission is of a large diameter, then it may also possibly contact the ground. Further, a difficulty arises in that the layout of the speed increasing mechanism with respect to the bicycle frame suffers reduced freedom. If the range of angular movement of the crank rod is changed to change the range of speed change ratios depending on the type of the bicycle or due to specification changes, then the crankshaft itself needs to be replaced, resulting in an increase in the cost of the continuously variable transmission.